ST style terminators provide a convenient method of terminating optical fibers in both digital and analog applications. The qualitative study of materials using light usually requires that a sample be illuminated and the light either reflected from or transmitted through the sample be analyzed. When an optical fiber is used to carry light to a sample, the light diverges rapidly as it exits the optical fiber tip and must be controlled and made into a collimated beam. When light has passed through or has reflected from a sample it must be made to converge, or focus, so that the light can be introduced into the small core at the tip of the optical fiber for efficient transmission to an analyzing instrument. In both cases, a focusing element is placed at a precise distance from the tip of the optical fiber. The distance from the optical fiber to the focusing element, as well as the concentricity of the optical fiber and the focusing element, are critical to ensure a reliable light signal for these applications.
Industry standard ST terminations are not designed to accurately control the position of the tip of the optical fiber. The main component of an ST terminator is a cylindrical ferrule mounted in a push and twist type mechanism, commonly known as a "bayonet" style connection. One end of the ferrule has an external shoulder and is constructed so that an optical fiber may be inserted. The other end has an opening that is smaller than the diameter of the ferrule, which forms an internal shoulder. Upon insertion, the tip of the optical fiber extends through the internal shoulder and registers axially inside the cylindrical ferrule flush with the outside face. Standard ST terminations rely on contact with the external shoulder of the connector, distal to the tip, for positioning. The ST termination process, during initial manufacture and during field repair, involves abrasively polishing the end of the ferrule having the internal shoulder and the optical fiber within. This process introduces an inconsistency in the final length of finished ferrules, as they extend from the external shoulder of the connector, of a significant fraction of a millimeter from connector to connector. Attempting to control axial position, distal to the optical fiber tip does not account for these variations and results in a similar variation in distance between the focusing element and the tip of the optical fiber. A means for precisely positioning the tip of any ST optical fiber terminator both axially for focusing and radially for concentricity would ensure consistency of optical focus and efficient light transmission with this common and convenient style of optical fiber connector.